Web Animations CSS: Support animation of -webkit-background-size and -webkit-mask-size

This patch extends animation of background-size introduced in
https://codereview.chromium.org/26259003/ to -webkit-background-size
and -webkit-mask-size which share the same logic.
The current animation implementation does not interpolate mismatched
values correctly and fails the newly added test.
The test passes under the Web Animations implementation.

BUG=257591

Review URL: https://codereview.chromium.org/27418005

git-svn-id: svn://svn.chromium.org/blink/trunk@159924 bbb929c8-8fbe-4397-9dbb-9b2b20218538